Abstract

YAL044 , a gene on the left arm of Saccharomyces cerevisiae chromosome one, is shown to code for the H-protein subunit of the multienzyme glycine cleavage system. The gene designation has therefore been changed from YAL044 to GCV3 to reflect its role in the glycine cleavage system. GCV3 encodes a 177 amino acid residue protein with a putative mitochondrial targeting sequence at its amino terminus. Targeted gene replacement shows that GCV3 is not essential for growth on minimal media. It is, however, essential for growth when glycine serves as the sole nitrogen source. Studies of GCV3 expression revealed that it is highly regulated. Supplement with glycine, the glycine cleavage system's substrate, induced expression at least 30-fold. In contrast, addition of the C1-metabolic end products repressed expression about 10-fold. The regulation of glycine cleavage system activity reflects the availability of glycine and the cellular demand for its metabolic products. In addition the glycine cleavage system has been shown to be important for the growth and viability of organisms ranging from microorganisms such as E. coli and S. cerevisiae to humans. Although, this system is important and its activity highly regulated little was known about the transcriptional regulatory mechanisms that control its activity. To address this I have examined the transcriptional regulation of the S. cerevisiae GCV3 gene. The results presented here show that at least six different transcriptional activators control GCV3 expression. These include: an as yet unidentified activator that is partially responsible for its induction by glycine; Gcn4p the transcriptional activator that mediates general amino acid control; Gln3p which is involved in the activation of nitrogen regulated genes; Gcr1p, a transcription factor important for the expression of glycolytic genes; Bas1p/Bas2p which cooperatively mediates the glycine-dependent expression; and an as yet unidentified factor that represses expression regardless of the growth condition. Additional evidence suggests that Rap1p, Nil1p, Acr1p, Ure2p, and Da180p also regulate GCV3 regulation.